The box structures of aircraft or other vehicles must contribute to the rigidity of the vehicle, and, for example, in the case of the central wing box of an aircraft, the structure is subject to major mechanical loads and must therefore have good fatigue strength in order to reduce the onset of cracks under load, especially in flight and on the ground. A fortiori, if the box is used as a fuel tank, it must have good static strength in case of sudden deceleration. Its mass, moreover, must remain low, even in the case of jumbo aircraft, and it must be easy to assemble.
The practice of placing rod lattices within the box structure as reinforcing ribs is known, and for this purpose these lattices are attached to the internal panels comprising the boxes.
The conventional construction of boxes provided with rod lattices is based on the rule of optimization of the number of rods which is based in turn on the principle of implementation of a triangularly braced isostatic system in which the rods and the segments of the panels between the rods are like bars, and the connecting points between the rods and the box panels and between the box panels are like nodes. The system is called isostatic and the number of bars is optimum when the relationship 2n−3=b is observed, in which n is the number of nodes and b is the number of bars.
In the case in which 2n−3 is less than b, the system is hyperstatically non-deformable, but there are b−(2n−3) excess bars.
In the case in which 2n−3 is greater than b, the system is hyperstatically deformable, but there are b−(2n−3) bars short.
Nevertheless, this definition is a theoretical rule and at it does not take into account the specific stresses on a box structure, such as the need to reinforce certain parts of the box, especially the front part of the box which can be subject to the impact of the fuel or objects contained in the box during an impact.